In history class, the girl to boy ratio is $5$ to $3$. If there are $12$ boys, how many girls are there?
A ratio of $5$ girls to $3$ boys means that there is a group of $5$ girls for every group of $3$ boys. If there are $12$ boys, then there are $4$ groups of $3$ boys each. Because we know the ratio is $5:3$ , there must be $4$ groups of $5$ girls each. There is a total of $20$ girls in history class.